As one key device for sharing multi-network in an indoor distribution system, a combiner mainly functions to combine signals from a plurality of communication systems into a single indoor distribution system. In the combiner, a fixed radio frequency channel is formed by two or more bidirectional filters, for allowing only frequency signals within passbands to pass through and blocking the passing of other frequency signals. Inevitably, there is certain loss in passband regions of the filters, and particularly larger loss at the edges of the passbands. The insertion loss of the combiner will influence the efficiency of the communication systems. Moreover, to ensure the normal operation of the mobile communication systems if they are co-sited, the isolation between ports of the combiner in the indoor distribution system needs to meet the requirements of related protocols. Therefore, the insertion loss and the isolation between ports of the combiner are important indicators of the performance of the combiner.